1. Field of the Invention
The present invention relates to an ink jet printing apparatus and a preliminary ejection method, and specifically, to control of preliminary ejection for a print head having ejection openings with which ink paths of different lengths communicate.
2. Description of the Related Art
Ink jet printing apparatuses are known to execute recovery processes. The recovery processes discharge ink of increased viscosity, fine bubbles, or the like in a print head and remove foreign matter, ink mist, or the like attached to a surface on which ejection openings are formed. The recovery processes thus allows an ink ejection characteristic of the print head to be appropriately maintained. Known examples of the recovery process include suction recovery, preliminary ejection and wiping. The suction recovery is an operation of utilizing a negative pressure generated by a pump or the like to suck ink or bubbles out of the ejection openings of the print head. The preliminary ejection is an operation of performing ink ejection irrelevant to image formation so that the ink is ejected to a predetermined place other than print paper or onto the print paper, to discharge the ink and bubbles via the ejection openings of the print head. The wiping is an operation of using a blade to wipe the surface on which the ejection opening in the print head are formed, to remove the foreign matter, ink mist, or the like attached to the ejection opening forming surface.
The above-described ink of increased viscosity in the print head may particularly cause a reduction of ejection amount of ink, a deflection of ejected ink, or an ejection failure such as non-ejection. The ink of increased viscosity is likely to result from the lack of ejection over a long period. An example of the conventional recovery processes first performs the suction recovery to suck the ink of increased viscosity out of the ejection openings in the print head and then carries out the preliminary ejection. This enables color mixture resulting from the suction recovery to be eliminated. The recovery processes using such suction recovery has room to be improved because the suction recovery results in a relatively large amount of waste ink. In contrast, though an intensity of recovery is lower than that of the suction recovery, the preliminary ejection involves a relatively small amount of waste ink. Thus, the amount of waste ink can be reduced by, for example, replacing the suction recovery with the preliminary ejection. Furthermore, the preliminary ejection process requires a shorter time than the suction recovery process and is thus prevented from significantly affecting printing throughput.
As described above, among the ejection recovery processes, the preliminary ejection is relatively effective in terms of the amount of waste ink and the printing throughput. However, driving conditions for the preliminary ejection are desirably optimized in order to effectively achieve the preliminary ejection. That is, the adverse effect of the ink of increased viscosity on the ink ejection depends on the color or type of the ink and the amount of ejected ink. Thus, the driving conditions such as the number of ejected ink droplets, ejection frequency, and ejection interval are desirably optimized to allow the above-described advantages of the preliminary ejection to be fulfilled.
Japanese Patent Laid-Open No. 06-246931 (1994) discloses a configuration that sets the number of droplets ejected during the preliminary ejection is set depending on the type of the ink. That is, Japanese Patent Laid-Open No. 06-246931 (1994) describes a process of achieving just enough preliminary ejection by first determining the type of the ink, reading the number of ejected ink droplets corresponding to the determined type of the ink and setting the read number, and then performing the preliminary ejection.
Also, Japanese Patent Laid-Open No. 2004-090292 discloses a configuration that sets the driving conditions for the preliminary ejection, specifically, the number of ejected ink droplets, the ejection frequency, and the ejection interval, depending on the amount of ejected ink. Specifically, Japanese Patent Laid-Open No. 2004-090292 describes a process of setting the appropriate driving conditions for the respective ejection openings of different sizes to minimize the time required for the recovery process and the amount of discharged ink.
In recent years, print heads have been provided which includes nozzle arrays in each of which ejection openings (hereinafter also referred to as nozzles) are densely arranged in order to provide high-quality color photo images. In such a print head, an increase in resolution per nozzle array is effective for reducing the chip size of the print head. However, a high resolution of about 1,200 dpi per nozzle array makes it difficult to linearly arrange, for example, electro-thermal conversion elements for generating ejection energy, in connection with manufacture of the print head. Thus, the ejection openings in each of the nozzle arrays are, for example, staggered so that the staggered arrangement as a whole achieves such high resolution. In such a nonlinear ejection opening arrangement, a distance from an ink supply port common to the ejection openings to each of the ejection openings, for example, the length of an ink path communicating with the ejection opening, depends on the arrangement of the nozzle. Consequently, this arrangement involves the plural types of ejection openings with the different distances.
Of course, the preliminary ejection is desirably also optimized for such a print head involving a plurality of distances from the ink supply port common to the ejection openings to the respective ejection openings. That is, the adverse effect of the ink of increased viscosity on the ink ejection is expected to vary depending on the shape of the ink path or the like, specifically, the distance from the ink supply port to the ejection opening. Thus, the driving conditions for the preliminary ejection are desirably optimized according to the level of the adverse effect.
The number of ejections (number of ejected ink droplets) in the preliminary ejection is associated with the time required for the recovery process and the amount of discharged ink, and is thus an optimization index. The number of ejections in the preliminary ejection can be reduced by decreasing the viscosity of the ink of increased viscosity. For example, conventional techniques apply short pulses that are insufficient to allow ink ejection or drive sub-heaters which are different from the electro-thermal conversion elements and which do not directly relate to the ink ejection, to increase the temperature of the ink to reduce the viscosity thereof. However, the addition of a new driving circuit or the sub-heaters leads to an increase in chip size. This disadvantageously increases the size or costs of the print head.
To solve this problem, the ink viscosity can be reduced by controlling, as a parameter, the ratio (E/Eth) of lower limit energy Eth supplied to the electro-thermal conversion elements and required for ejection to energy E supplied for actual ejection. That is, setting the ratio E/Eth to be equal to or greater than a certain value is known to be effective for increasing the temperature of the ink to reduce the viscosity of the ink of increased viscosity